With a view to generating various colors including white uniformly with a good color rendering property by mixing blue, red and green colors, a light emitting device which radiates colored light from a first light emitter such as LED or LD to a phosphor as a second light emitter to cause color conversion is proposed. For example, in Japanese Patent Publication No. Sho 49-1221, proposed is a method of forming a display by exposing a phosphor (Y3-x-yCexGdyM5-zGazO12 (in which Y represents Y, Lu or La, and M represents Al, Al—In or Al—Sc) to the beam of a laser (first light emitter) which radiates a beam with wavelengths of from 300 to 530 nm and thereby causing its light emission.
In recent days, a white-light emitting device having, as a first light emitter, a gallium nitride (GaN) LED or LD which has attracted attentions as a blue light emitting semiconductor light emitting element and having a high emission efficiency and, as a second light emitter, a variety of phosphors is proposed as a radiation source for an image display device or illuminator by making use of its features such as low power consumption and long life.
For example, described in Japanese Patent Application Laid-Open No. Hei 10-242513 is a light emitting device having, as a first light emitter, a nitride semiconductor LED or LD chip and, as a second light emitter, a yttrium•aluminum•garnet phosphor. In U.S. Pat. No. 6,294,800, a substance having a green-light emitter containing Ca8Mg(SiO4)4Cl2:Eu2+,Mn2+, a red light emitting phosphor and a blue light emitting phosphor in combination is disclosed as a substance capable of generating white light when exposed to light in a range of from 330 to 420 nm, that is, typified by light from LED. As an example of the blue light emitting phosphor, (Sr,Ba,Ca)5(PO4)3Cl:Eu2+ is mentioned. In U.S. Pat. No. 6,278,135, described is a light emitting device which emits visible light when a phosphor is exposed to ultraviolet light from LED. As the phosphor, BaMg2Al16O27:Eu2+ is given.
The above-described light emitting devices using such phosphors have not emission intensity enough for use as a light emitting source for displays, backlight sources or traffic signals. There is accordingly a demand for further improvement.
When a light emitting device using a first light emitter and a second emitter in combination is employed as a light emitting source for displays, backlight sources and traffic signals, it is required to have high emission intensity and at the same time, to be excellent in a color rendering property which serves as a measure for showing how the appearance of an object exposed to white light from a light emitting device resembles to that exposed to sunlight.
For example, as described in Japanese Patent Application Laid-Open No. Hei 10-242513, when a blue LED or blue laser as a first light emitter is used in combination with a cerium-activated yttrium aluminum garnet phosphor as a second light emitter, a white color is generated by mixing the blue light generated from the first light emitter with a yellow color generated by the second light emitter. When the emission spectrum produced by this combination is compared with the spectrum of sunlight, emission intensity is considerably low in an intermediate region (470 nm to 540 nm) between the blue light emission peak top (near 450 nm) and yellow light emission peak top (near 550 nm), and in a long wavelength region (580 to 700 nm) of a yellow peak. A blue•yellow mixed color system light emitting device has, in its emission spectrum, a low emission intensity portion different from the spectrum of a sunlight, so that its color rendering property is very low.
On the other hand, a white light emitting device using, as a second light emitter, a mixture of blue, green and red phosphors is expected to have an improved color rendering property, because three peaks overlap each other different from the conventional blue•yellow mixing system in which two peaks overlap and a low emission intensity portion between peaks is narrowed. Even this blue•green•red mixed system, however, has a low emission intensity portion when half-width of each emission peak is small so that the problem of a low color rendering property still remains unsolved.    [Patent Document 1]
Japanese Patent Publication No. Sho 49-1221    [Patent Document 2]
Japanese Patent Application Laid-Open No. Hei 10-242513    [Patent Document 3]
U.S. Pat. No. 6,294,800    [Patent Document 4]
U.S. Pat. No. 6,278,135
With the above-described background art in view, the present invention has been made to develop a light emitting device having a considerably high emission intensity and good color rendering property. An object of the present invention is therefore to provide a double light emitter type light emitting device which can be manufactured easily, has a high emission intensity and has improved color rendering property.